Abstract
The closure process after shut-in of the cracks which are induced by hyraulic fracturing on the surface of a wellbore is analyzed to discuss the physical meaning of shut-in curves. The analysis is based on the linear theory of elasticity and also on linear fracture mechanics. It is shown that the pressure decay characteristics of shut-in curves are primarily governed by the leak-off rate, the equipment compliance and the partial crack closure. It is also shown that the downhole fluid pressure at the onset of the closure of the crack tip is very close to the compressive tectonic stress normal to the crack plane. This pressure can be detected as the point of maximum curvature on each shut-in curve. Finally, the methods which have been proposed so far for the determination of instantaneous shut-in pressures are also applied and their applicability and reliability are discussed.
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